Hybrid t-junction constructed in waveguide having a cut-off frequency above the operating frequency

ABSTRACT

A hybrid T-junction constructed in waveguide for operation below its cut-off frequency. The series arm of the hybrid provides a parallel resonant circuit in series with the symmetrical arms of the hybrid. The shunt arm provides a series resonant circuit at the midpoint of the series arm. The equivalent circuit is a lumped circuit bridged-T, which exhibits the properties of balance and isolation associated with bridge networks.

United States Patent Craven et al.

1 1 3,657,668 [4 1 Apr. 18, 1972 HYBRID T-JUNCTION CONSTRUCTED INWAVEGUIDE HAVING A CUT-OFF FREQUENCY ABOVE THE OPERATING FREQUENCYInventors: George Frederick Craven; Raymond Richard Thomas, both ofHarlow, England Assignee: International Standard Electric Corporation,New York, N.Y.

Filed: May 25, 1970 Appl. No.: 40,074

Foreign Application Priority Data June 6, 1969 Great Britain.......28,698/69 U.S. Cl. ..333/1l, 333/73 R, 333/28 R Int. Cl. ..HOIp 5/12Field of Search ..333/l1, 28 R, 98 BE ADJUS MBL E CA PA 6/ 7/ V5 TUNINGSC/QE W References Cited UNITED STATES PATENTS 2,685,065 7/1954Zaleski... 333/11 X Primary Examiner-Paul L. Gensler Attorney-C. CornellRemsen, Jr., Walter J. Baum, Paul W. Hemminger, Charles L. Johnson,.lr., Philip M. Bolton, lsidore Togut, Edward Goldberg and Menotti J.Lombardi, J r.

[5 7] ABSTRACT A hybrid T-junction constructed in waveguide foroperation below its cut-off frequency. The series arm of the hybridprovides a parallel resonant circuit in series with the symmetrical armsof the hybrid. The shunt arm provides a series resonant circuit at themidpoint of the series arm. The equivalent circuit is a lumped circuitbridged-T, which exhibits the properties of balance and isolationassociated with bridge networks.

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A Home y HYBRID T-JUNCTION CONSTRUCTED IN WAVEGUIDE HAVING A CUT-OFFFREQUENCY ABOVE THE OPERATING FREQUENCY BACKGROUND OF THE INVENTION Thisinvention relates to electrical waveguide arrangements, and particularlyto an evanescent mode waveguide hybrid T- junction.

SUMMARY OF THE INVENTION According to the broadest aspectof theinvention there is provided a waveguide hybrid T-junction constructed inwaveguide having a cut-off frequency above the operating frequency,comprising a rectangular main waveguide forming first and secondsymmetrical arms of the junction dimensioned to have a cut-off frequencyabove the operating frequency, a first branch rectangular waveguide,dimensioned to have a cut-off frequency above the operating frequency,directly coupled into a first wall of said main waveguide forming aseries arm to provide a parallel resonant circuit at the operatingfrequency and a second branch retangular waveguide dimensioned to have acut-off frequency above the operating frequency, directly coupled to asecond wall of said main waveguide forming a shunt arm to provide aseries resonant circuit at the operating frequency at the midpoint ofsaid series arm.

The invention will be described with reference to the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of anevanescent mode waveguide hybrid T-junction,

FIG. 2 is a basic schematic diagram of a known form of hybrid junction,

FIGS. 3 and 4 are equivalent circuits of the junction of FIG. 1

FIGS. 5 to 12 are equivalent circuits of alternative forms of evanescentmode resonators,

FIG. 13 is an equivalent circuit of the junction of FIG. 1,

FIGS. 14 and 15 are all-pass filter networks,

FIGS. 16 and 17 are equivalent circuits of single-section andmulti-section phase equalizers respectively using an evanescent modehybrid junction and junctions,

FIG. 18 is a perspective view of a multi-section phase equalizer,

FIG. 19 shows part of a known form of phase equalizer,

FIG. 20 is a perspective view of an evanescent mode phase equalizer,

FIGS. 21 and 22 are perspective views of alternative forms of anevanescent mode waveguide series (E) T-junction,

FIGS. 23 and 24 are perspective views of alternative forms of anevanescent mode waveguide shunt (H) T-junction,

FIGS. 25 and 26 are perspective views of alternative forms of anevanescent mode waveguide right angle (E) bend, and

FIGS. 27 and 28 are perspective views of alternative forms of anevanescent mode waveguide right angle (H) bend.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, arectangular main waveguide 1 has a first branch rectangular waveguide 2directly connected into one side wall, and a second branch rectangularwaveguide 3 directly connected into one broad wall, the branch 3 beinglocated transversely to the main guide 1 symmetrically about the broadwall center line of the branch 2.

Located on the broad wall longitudinal center line of the guide 1 areadjustable capacitive screws4 and 5, each at the mid-point of the lengthl of the respective portion of the guide 1 on either side of the branchjunction and each extending into the respective portion of the guide.

There is an adjustable capacitive screw 6 located at the junction planeof the branch 2 with the guide 1 and extending into the branch 2.

Located in the. plane of the junction between the branch 3 and the guide1 and completely filling the junction aperture is a thin dielectricplate 7 which constitutes a capacitive obstacle. An adjustabledielectric screw 8 moveable in or out of a corresponding threadedtransverse aperture 9 in the plate 7 may be provided for tuningpurposes.

Located on the broad wall longitudinal center line of the branch 3, at adistance 1 from the junction plane and the dielectric plate 7 is anadjustable capacitive screw 10 extending into the branch 3.

Located at the center of the junction, i.e. at the intersection point ofthe guide land the branch 2 center lines, is an adjustable capacitivescrew 11.

The waveguide arrangement shown in FIG. 1 is designed to function as anevanescent mode hybrid T-junction, and accordingly all the guide isdimensioned to have a cut-off frequency above the required operatingfrequency.

As is well known, dominant mode waveguide ceases to propagateprogressive waves below its cut-off frequency, and the mode is said tobe evanescent. Waveguide in which the dominant mode is evanescent has apositive imaginary (inductive) characteristic impedance 2,) to anincident TE mode and a real propagation constant (7), and thereforebehaves es sentially as a pure reactance. If a short section of thisguide is terminated in an obstacle which presents a conjugate(capacitive) reactance at a frequency below the cut-off frequency, theincident power at that frequency will be completely transmitted throughthe section.

This full transfer of energy through evanescent waveguides is more fullydescribed in Waveguide Bandpass Filters Using Evanescent Modes", G.F.Craven, Electronics Letters, Vol. 2 No. 7 July 1966, pp. 25-26, and inBritish Patent Specification No. 1,129,185.

It will be apparent, therefore, that in "the waveguide arrange mentshown in FIG; 1, full energy transfer through any one of the four armsof the junction is basically achieved, in the evanescent mode, bysuitably adjusting the respective capacitive obstacle or obstaclesassociated with thatparticular arm to obtain the required conjugatematch condition.

However, for the arrangement to function as a hybrid T- junction, thenecessary requirements of balance and matching must also be met.

How this is achieved in the present embodiment is most readily describedby first considering certain basic aspects of a known form of hybridjunction constructed in propagating guide.

The basic requirement for a four-port matched hybrid junction is a typeof lattice (bridge) network which in its more restricted form consistsof a bridged-T network. One example of a network with these properties,realized in propagating guide, is a resonant slot hybrid junction whichhas been fully described in Resonant-slot hybrid junctions and channeldropping filters G.F. Craven, D.W. St-opp and RR. Thomas, Proc. I.E.E.,Vol. 112, No. 4, Apr. 1965, pp. 669-680, and British Patentspecifications 987,593 and 1,053,071.

In this resonant slot hybrid junction two of the conjugate arms areprovided by a length of main rectangular waveguide, and each of theother two arms is provided by a branch waveguide joined to a broad wallof the main waveguide. There is a transverse resonant slot in the mainwaveguide providing coupling between the first mentioned two arms andone of the other arms, and a longitudinal resonant slot in the mainwaveguide providing coupling to the remaining other arm.

This can be represented as shown in FIG. 2, with the main rectangularpropagating waveguide length 20 having a transverse (series) resonantslot 21 and'a longitudinal (shunt) resonant slot 22. The two branchguides, one to each slot, are not shown.

The equivalent circuit of this configuration is shown in FIG. 3, withits reduction to the bridged-T of FIG. 4.

I The shunt and series slots are in the same reference plane, and thetransverse (series) slot 21 couples only to the transverse component ofthe magnetic field; the longitudinal (shunt) slot 22 coupled only to thelongitudinal component of the magnetic field. As a result of thesymmetry associated with the configuration, the two slots do not coupleto each other. Although the two slots are identical parallel resonantslots, the longitudinal slot 22 appears at its reference plane in themain guide as a series resonant circuit.

The explanation of this is that the phase of the longitudinal componentof the field lags the transverse component by 90 Thus the equivalentcircuit of FIG. 3 shows an equivalent quarter-wave line (M4) whichinverts the parallel resonant circuit to a series resonant circuit inthe main guide.

The necessary conditions for matching the junction are derived frominspection of FIG. 3. The impedance connected to the series transformerterminals is 22 so that for a match.

Similarly the impedance connected across the shunt transformer terminalsis Z /2 and for a match 01/ o1 n3' ip' 2n For operation of thearrangement shown in FIG. 1 as an evanescent mode hybrid T-junction, itwill later be shown that the branch guide 2 functions as the shunt armand that the guide 3 functions as the series arm.

There is symmetry between the shunt arm 2 and the series arm 3 of thejunction and therefore isolation exists between these arms. However apropagating mode does not exist in the guide (by definition) and as aresult the two fields exciting the series and shunt arms are in phase.Thus no impedance inversion of the shunt network occurs. The problem ofrealizing the necessary dual elements for the respective series andshunt arms can be resolved in the following way.

Two types of evanescent mode resonator are possible 11' section and Tsection. The classification follows from either a capacitance at eachend of a length I of evanescent guide (11 section), or one capacitancein the center of a length l of evanescent guide (T section). These twotypes are shown in FIGS. 5 and 6 respectively, and their equivalentcircuits in FIGS. 7 and 8.

These equivalent circuits can be shown in a different way involving theconcept of impedance inverters. This concept is in itself well known,being described for example in Transmission Networks and Wave Filters"T.E. Shea, p. 329, and involves ideal transformers using negativeelements (which in practice are absorbed in a positive element) toprovide networks which are essentially broadband impedance invertingnetworks.

FIGS. 9 and show the circuits of FIGS. 7 and 8 respectively redrawn toinclude such a network, the impedance inverting networks being enclosedin solid rectangles.

In FIGS. 9 and 10 the resonant elements are effectively connectedtogether by equivalent quarter-wave lines. Thus the 11' sectionresonator appears as shown in FIG. 11, and this is the type of circuit(parallel resonant) required for the series arm 3 It can be demonstratedthat the image impedance, Z,, of the network enclosed in the rectangleof FIG. 9 is given by Z, Z sinh yl and the phase constant of the networkis given by It will be seen that the network is equivalent to atransmission line with a resistive characteristic impedance (whichvaries with frequency) but which is a quarter wave long at allfrequencies.

The T section resonator appears as shown in FIG. 12, and seen from itsinput terminals behaves as a series resonator. This is the type ofcircuit required for the shunt arm 2. A second characteristic of theseries resonator of FIG. 12 is the shunt element (shown within thedashed outline rectangle) which is the remnant of the original sectionof FIG. 10 when it is modified to include the impedance inverter at theinput terminals.

With a load resistance of R, the equivalent source impedance consists ofthe load resistance and the remnant, which is an inductive reactance Ztanh in parallel.

The equivalent source impedance, 2,, expressed in series form is then IThis shows that the equivalent series source impedance is reduced by theinductive reactance of the shunt element. If this reactance iscontrolled by a shunt capacitance, the source resistance can be variedand matched to the load impedance.

Returning now to FIG. I, it has been demonstrated that for thearrangement to function as a hybrid T-junction, the required seriesresonant circuit at the mid-point of the series arm 3 is realized by theshunt arm 2, and the required parallel resonant circuit is realized bythe series arm 3, in series with the symmetrical arms 1.

FIG. 13 shows the equivalent circuit of the evanescent mode hybridT-junction as a bridged-T network, analogous to the bridged-T networkshown in FIG. 4, but representing the impedance of the main waveguide 1as functions of the inductive characteristic impedance (2 of evanescentwaveguide, y, and the length I, of each portion of the guide 1 on eachside of the junction.

The matching parameters of the series arm 3 are the tuning of theparallel resonant circuit (by screw 8 and/or screw 10) at the inputterminals to control the reactive component, and the suitable selectionof the length 1 to control the impedance transformation.

The matching parameters of the shunt arm 2 are similar. The capacitivescrew 6 is tuned to control the reactive component of the seriesresonant circuit, and the length 1 is selected to control the impedancetransformation. Minor fine adjustment can be made by the screw 11 toprovide a variable source impedance to the shunt arm as alreadydescribed.

The screws 4 and 5 are tuned to give the earlier mentioned conjugatematch condition for full energy transfer through the respectiveportions.

It will be appreciated that each of the main waveguide portions I,instead of containing a single centrally located capacitive screw, mayeach contain two spaced capacitive obstacles, one at each end of thelength 1,.

Although capacitive screws have been described for obtaining thenecessary capacitive reactances, it is to be understood that anysuitable means of obtaining the required capacitive reactances may beused.

The characteristics of an all-pass filter or all-pass network areillustrated in the lattice or bridge network of FIG. 14 and FIG. 15.Such a network transmits energy from zero frequency to infinitefrequency, and with correct choice of component values no amplitudechange occurs. The phase of the input to output voltage is, however, afunction of frequency, as is evident from the extreme conditions; atzero frequency cur rent takes the ADCB path, whereas at infinitefrequency current takes the ACDB path. Thus the network has a maximumphase change of All-pass filters of this type are used to correct thephase characteristics of conventional low-pass filters. By taking afurther step, FIG. 16, to introduce series and parallel resonantcircuits, the network can be used as a correction network for bandpassfilters. This general lattice structure can be reduced to a bridged-Tnetwork, and therefore the structure of FIG. 4, and FIG. 13, isapplicable to all-pass network applications.

In this application a single evanescent mode waveguide hybrid T-junctionas an all-pass phase equalizer network could appear as shown in FIG. 16.The basic conditions that must be satisfied by the network for it to bereflectionless have been derived in the paper Resonant slot hybridjunctions and channel dropping filters already detailed. The series andparallel resonant circuits must have equal loaded Q-factors. Thus if QpQ:

Q, 2wL,/R where R source impedance and The realization of the equivalentresonant circuits with evanescent mode resonators has already beendescribed. The basic design variable is the cavity length I. Z, Z sinh-yl) is used as a variable and the L/C ratio treated as a constant.

A multi-section phase-equalizer is shown in FIG. 17, and is realizedeither by directly coupling together in cascade the main waveguideportions of a corresponding number of hybrid junctions each as shown inFIG. 1, or by an integral structure as shown in FIG. 18, in which thereis an effective merging of output-input main arms of successive hybridjunctions into a single common arm, and like references to FIG. 1 havebeen used.

In either the directly coupled arrangement of individual junctions orthe integral structure, the input for phase-correction is applied to oneend main guide 1A and the corrected output derived from the other end IDof the main guide, and each series and shunt arm 2 and 3 is terminatedby a short circuit (not shown).

The phase equalization of bandpass filters using hybrid junctions ortheir equivalents has been described for example in Development ofgroup-delay equalizers for 4 Gc/s, D. Merlo, Proc. I.E.E., Feb., 1965.

In propagating guide the equalizer is realized with two identicalbandpass filters 30 each in the two shortcircuited (31) arms in the wayshown in FIG. 19. In evanescent guide the equivalent quarter wave shiftcan be realized as shown in FIG. 20 by having one main arm 1 of thehybrid junction comprising a series resonant ('l" section) cavitycontaining a single central capacitive screw 5, and the other main arm 1comprising a parallel resonant (11' section) cavity containing acapacitive screw 4 located at the end of the section remote from thejunction and a dielectric plate 40, which may have an adjustabledielectric screw 41, filling the guide section at the junction planebetween the end of the section and the broad wall of the series arm 3.

Each of the main arms then couple into identical multi-section bandpassfilters (not shown) terminated by a short circuit. Input is the shuntarm 2, and output the series arm 3.

Thus in practical realization the bandpass filters integrate into thehybrid junction assembly.

Tjunctions both series (E) and shunt (H) are necessary components inmany systems. Either type of junction can readily be realized inevanescent guide because either junction is merely a special case of anevanescent mode hybrid junction in which one of the isolated arms (shuntor series) is short circuited at its junction with the other arms.

In the same way, a right angled bend can be considered as a special caseof a T-junction with one of its arms short cir-' cuited.

FIGS. 21 and 22 show alternative forms of an evanescent mode series (E)waveguide T-junction, with, in FIG. 21, three T-section evanescent moderesonators, each of length 1, between capacitive screw 4 and dielectricplate 7, between capacitive screw 5 and dielectric plate 7, and in theseries arm 3 between dielectric plate 7 and capacitive screw 10.

In FIG. 22 there are three 1r section evanescent mode resonators, eachof half length 1/2, provided respectively between dielectric plates 40A,40B and 7.

FIGS. 23 and 24 show alternative forms of an evanescent mode shunt (H)waveguide T-junction. In FIG. 23 there are three T-section evanescentmode resonators, each of length 1, between capacitive screws 4 and 11,between capacitive screws 5 and 11, and in the shunt arm betweencapacitive screws 6 and 11.

In FIG. 24 there are three 11' section evanescent mode resonators, eachof half length 1/2, formed respectively between dielectric plates 40A,40B and 6.

FIGS. 25 and 2 show alternative forms of an evanescent mode waveguideright angle (H) band. In FIG. 25 there are two 71' section evanescentmode resonators each of half length l/2 formed respectively betweendielectric plate 40A and 7. In FIG. 26 there are two T-sectionevanescent mode resonators, each of length 1, formed between capacitivescrews l0, l1 and 4 FIGS. 27 and 28 show alternative forms of anevanescent mode right angle (E) band. In FIG. 27 there are two 11'section evanescent mode resonators, each of half length 1/2, formedrespectively between dielectric plates 40 and 6.

In FIG. 28 there are two T-section evanescent mode resonators, each oflength 1, formed between capacitive screws 4, 6 and l l.

I claim:

1. A waveguide hybrid T-junction constructed in waveguide having acut-off frequency above the operating frequency, comprising:

a rectangular main waveguide forming first and second symmetrical armsof the junction dimensioned to have a cutoff frequency above theoperating frequency;

a first branch rectangular waveguide dimensioned to have a cut-offfrequency above the operating frequency, directly coupled into a firstwall of said main waveguide forming a series arm to provide a parallelresonant circuit at the operating frequency;

a second branch rectangular waveguide, dimensioned to have a cutofffrequency above the operating frequency, directly coupled to a secondwall of said main waveguide forming a shunt arm to provide a seriesresonant circuit at the operating frequency at the midpoint of saidseries arm;

a first capacitive obstacle located at the center of the length of saidfirst symmetrical arm;

a second capacitive obstacle located at the center of the length of saidsecond symmetrical arm;

two capacitive obstacles each located at opposite ends of said seriesarm; and

a third capacitive obstacle located at the junction of said first andsecond symmetrical arms.

1. A waveguide hybrid T-junction constructed in waveguide having acut-off frequency above the operating frequency, comprising: arectangular main waveguide forming first and second symmetrical arms ofthe junction dimensioned to have a cut-off frequency above the operatingfrequency; a first branch rectangular waveguide dimensioned to have acutoff frequency above the operating frequency, directly coupled into afirst wall of said main waveguide forming a series arm to provide aparallel resonant circuit at the operating frequency; a second branchrectangular waveguide, dimensioned to have a cut-off frequency above theoperating frequency, directly coupled to a second wall of said mainwaveguide forming a shunt arm to provide a series resonant circuit atthe operating frequency at the midpoint of said series arm; a firstcapacitive obstacle located at the center of the length of said firstsymmetrical arm; a second capacitive obstacle located at the center ofthe length of said second symmetrical arm; two capacitive obstacles eachlocated at opposite ends of said series arm; and a third capacitiveobstacle located at the junction of said shunt arms and said first andsecond symmetrical arms.